It is well known that in many microwave applications, improved performance is achieved by minimizing losses in the transmission media. This is especially true for critical microwave system components like low noise amplifier (LNA) input matching circuits, power amplifier (PA) output matching circuits, power combiners, and high Q filters. In these critical applications, losses are the primary factor in achieving suitable performance.
It is also well known that Gallium Arsenide (GaAs), which is commonly used as a substrate for fabricating monolithic microwave integrated circuits (MMICs), has relatively high loss compared to other available microwave substrate materials. For this reason, in applications such as those mentioned above where minimizing loss is important, the critical circuit is fabricated on a lower loss substrate such as alumina or quartz, and then interconnected with the rest of the MMIC. For example, power amplifiers are often designed with off-chip output matching circuits. It is also known that even lower loss can be achieved by suspending the substrate away from top and bottom ground planes. This is known as suspended stripline transmission media. Suspended stripline is sometimes used in filters where higher Q is needed and in other critical applications.
At higher frequencies it becomes more difficult to fabricate a low loss transition between the MMIC and the off-chip circuitry. In addition, slight variations in such an interface add parasitic reactance to the circuit. At millimeter wave frequencies, this reactance can cause mismatches that degrade performance even more severely than the losses encountered by keeping the circuit on-chip. Even extensive manual tuning of the interface circuit may not improve the interface mismatch problem.
Therefore, what is needed is a method of improving circuit performance by reducing the loss of critical on-chip MMIC circuitry. What is also needed is a method of avoiding the reactive mismatch problem of high frequency transitions and improving MMIC circuit performance without tuning. Further needed is a method of achieving low loss on MMIC circuits compatible with existing MMIC fabrication processes.